The difference between the two colors of the Jaerhon is at the E locus which controls the base color of the chicken. It is the same difference as the difference as is found between the brown and light brown Leghorn. Most people can not accurately tell the adults apart. The surest way to tell the Light Jaerhon from the Dark Jaerhon is the color of the chick down. The Dark Jaerhon is genetically eb, dark brown. The Dark chicks are brown. The Light Jaerhon is genetically e, wild type. The light chicks have chipmunk stripes on their backs. If you want to separate the two types, which I think is great, then do so right after hatch when you separate the pullets from the roos. If you don't separate them then mark them in some permanent way such as toe punches.